Conventionally, a steering control system with a rotating electrical machine is used as a driving source of a power steering apparatus of a vehicle. For example, JP-A-2009-1217 discloses an electric power steering apparatus that uses a rotating electrical machine driven by a pulse width modulation (PWM) control as a driving source. The rotating electrical machine is a brushless motor having windings of three phases. The rotating electrical machine is driven by a rotating electrical machine control device including a switching element.
It is general in a rotating electrical machine control apparatus performing a PWM control that when driving of a brushless rotating electrical machine including windings of a plurality of phases is stopped, duty ratios of voltages applied to respective windings are controlled to be 50% and an electric current supplied to the rotating electrical machine is controlled to be zero by applying the voltages to the windings of the respective phase at the same timing. Even when the electric current supplied to the rotating electrical machine is controlled to be zero, an electric current of a common mode (hereafter, referred to as a common mode current) may flow in a spike shape due to a parasitic capacitance of the rotating electrical machine when an application of the voltage is started or stopped. Especially, when the voltages are applied to the windings of the respective phases at the same timing, the common mode currents may overlap each other to be one spike-shaped large current. Accordingly, large electric noise may be output as radio noise and may affect other apparatus.
In the electric power steering apparatus in JP-A-2009-1217, the rotating electrical machine control device includes a filter circuit for reducing the radio noise. As other methods for reducing the radio noise, a snubber circuit or a shielding wire may be added, or a coaxial cable may be used as disclosed in JP-A-11-29054. In the above described methods, because the number of components is large and a configuration is complicated, a cost for reducing the radio noise increases.